There is considerable interest in organic materials with large second order hyperpolarizabilities for use in nonlinear optical (NLO) devices such as modulators and frequency doublers. To achieve a high figure of merit for such devices requires a material with a non-centrosymmetric bulk crystal structure and low dielectric constant. To this end, NLO-active chromophores are traditionally incorporated into a polymer matrix and electrically poled to achieve the necessary bulk symmetry. However, such materials are limited by their low glass transition temperatures and poor stabilities at elevated temperature.
Recently, single crystals of organic and organometallic salts have been shown to possess extremely large second order (X.sup.(2)) NLO effects leading to a high second harmonic generation (SHG) efficiency. The naturally non-centrosymmetric crystal structures of these compounds obviates the need for external poling. Furthermore, these salts have a high optical damage threshold and sufficient stability with respect to temperature to withstand many conventional semiconductor fabrication processes. In particular, highly pure single crystals of the salt 4'-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) have been shown to have a value of X.sup.(2) at least 10.sup.3 times greater than that of urea due to dipole alignment of the cation and anion constituents of the DAST structure. To illustrate this alignment, the SHG-active DAST bulk crystal structure is shown in FIG. 1A.
For many applications such as waveguide devices, it is desirable to fabricate NLO materials into optical quality thin films. Although thermal evaporation in a high vacuum environment has been used to grow thin films of many organic and inorganic materials, the technique is not always applicable to organic salts. For example, when heated in vacuum, DAST decomposes before entering the vapor phase. Attempts at double-source co-evaporation of DAST neutral precursors 4'-dimethylamino-4-stilbazole (DAS) and p-methyl tosylate (MT) to form DAST have also been unsuccessful, due in part to the radically different vapor pressures of DAS and MT, which leads to highly incongruent growth.
Vapor Phase Epitaxy (VPE) has been used to grow epitaxial thin films of many group III-V compound semiconductors, such as InP and GaAs, where there is a large difference in the vapor pressures of the group III and group V atomic constituents.